1. Field of the Invention
The invention in general relates to hydrophones, and in particular to a dipole hydrophone with a low vibration sensitivity, a high acoustic sensitivity and a low flow noise response.
2. Description of the Prior Art
Dipole hydrophones find extensive use in the underwater environment for listening to very low frequency noise as may be produced for example, by a submarine. The dipole hydrophone is positioned at some point in the water, either alone or as a part of an array, and provides an output signal in response to received acoustic signals in accordance with its beam pattern in the form of a figure eight.
Most dipole hydrophones respond directly to particle velocity and any mechanical vibration acceleration from the support structure may tend to provide an unwanted output signal.
In copending application Ser. No. 352,820, filed Apr. 19, 1973, and assigned to the same assignee as the present invention, there is described a dipole hydrophone which utilizes two masses having different ratios of actual mass to added radiation mass with each being connected by means of a multi-laminar magnetostrictive arm to a base member, with the unit including a number of pickups for providing an output signal. This hydrophone significantly reduces the effects of acceleration, however, it does require two matched multi-laminar arms and two matched pickup units.
To eliminate the particle velocity response, a dipole hydrophone has been proposed which responds to the pressure gradient of an acoustic wave by means of two monopoles separated by a half wavelength and connected so that the signals from the monopoles subtract. Although the arrangement has very desirable inertia balancing properties, there are disadvantages. For example, the sensitivity is limited by the thermal noise of the preamplifiers utilized in the signal processing. A difference signal may be extremely small compared with this thermal noise. Further, in order to obtain an accurate output, the monopoles and signal processing channels must be very accurately balanced.
In a somewhat analogous art, a pressure gradient microphone has been proposed which includes a housing containing a differential pressure sensor and includes elongated first and second arms extending from the housing to spaced apart points where the respective pressures are communicated to either side of the differential pressure sensor. Such arrangement, to be described in FIG. 2, is air or gas filled and has a high acoustic sensitivity with low response to flow noise. The arrangement, however, is not suitable for underwater use; however, even if filled with a liquid and operated underwater, the unit would be extremely sensitive to vibrations.